In large steam turbines used in power generation, steam leaving the last row of turbine blades flows through an annulus between the turbine enclosure or casing and the bearing cone into a collector called the "exhaust hood" and then to a condenser. The vast majority of exhaust hoods of condensing steam turbines are of the "downward-discharging" type in which the condenser is located below the exhaust hood. Although the following description is applicable particularly to turbines with "downward-discharging" types of exhaust hoods, it applies broadly to other types of turbines.
At the outer radius of the last turbine blade annulus there is usually located a fixed, outer guide vane of circumferentially uniform shape which extends for 360.degree. with respect to the turbine axis of rotation. The fixed, outer guide vane is usually made of two 180.degree. segments, one placed above and the other placed below the horizontal plane of the turbine shaft longitudinal axis. The inner surface of the outer guide vane and the outer surface of the turbine bearing cone form an exhaust flow passage in which the steam passing from the last turbine blades is, preferably, decelerated or, in other words, diffused. The diffusion causes a decrease in kinetic energy of the steam and a corresponding increase in pressure from the last turbine blades to the exhaust flow passage exit. This exhaust flow passage exit pressure is influenced by the pressure in the condenser located after the exhaust hood. Since with diffusion there is an increase, in the flow direction, of steam pressure in the exhaust flow passage, there is a corresponding decrease in pressure of the steam at the exit of the last row of turbine blades below the exhaust hood pressure and a corresponding increase in turbine work output as compared to the work output which would occur in the absence of diffusion.
The average exhaust hood pressure at the exhaust flow passage exit is close to the pressure in the condenser following the exhaust hood. However, the average exhaust hood pressure is somewhat higher because of the pressure losses that occur due to struts and beams located, for reinforcing purposes, throughout the exhaust hood, some being near the condenser flange and in the condenser neck. For a given condenser pressure, which depends on the temperature of the condenser cooling water, the higher is the pressure rise caused by the diffusion process in the exhaust flow passage, the lower is the pressure at the exit of the last row of turbine blades and the higher is the turbine work output for given turbine inlet conditions.
The pressure in an exhaust flow passage varies circumferentially relative to the turbine axis. Because of internal pressure losses, the pressure in the top portion of a downwardly-discharging exhaust hood is higher than in the bottom portion, which is closer to the condenser inlet. At the same time, the level of pressure in the exhaust hood is affected by the condenser pressure which depends on the cooling water temperature and on the steam flow rate. In the summer the cooling water temperature in the condenser and the corresponding condenser pressure are higher than in the winter. An outer guide vane having a fixed shape designed for flow conditions at the exit of the last row of turbine blades prevailing during one season, e.g. winter, will not perform as well in a different season, e.g. summer. Also, if the turbine load and with it the steam flow rate were to decrease below the design load, a fixed guide vane designed for the full load would not perform well.
At present, no matter what type of exhaust hood is used for a turbine, manufacturers of such equipment make the exhaust end outer guide vanes fixed and in general of uniform shape; that is the shape of the guide vane is fixed and, in general, it does not vary circumferentially from one position to another. Generally, the guide vanes are designed to perform well at flow conditions corresponding to very low condenser pressures, usually those corresponding to winter conditions and full load. Consequently, such fixed guide vanes do not perform equally well for the high condenser pressures which occur during the summer months. In addition, in downwardly-discharging exhaust hoods, uniformly shaped guide vanes do not provide optimum exhaust flow passages for the varying steam flow conditions around the last blade annulus. The flow conditions vary because, as has been mentioned already, as a consequence of the pressure losses, the pressure at the top portion of such exhaust hoods is higher than in the bottom portion. Present exhaust steam turbines have fixed exhaust guide vanes designed to operate at certain selected conditions. Heretofore no effort has been made to change the shape of the exhaust flow passages in response to varying conditions of turbine operation by making the guide vanes adjustable or by any other modification.